Re: guix gc: smarter collection & guix.el manual deletion

[Chris Marusich]

Pierre Neidhardt <address@hidden> writes:

> Thanks for mentioning this.  I'm very confused by the documentation however:
>
> ‘--gc-keep-outputs[=yes|no]’
>      Tell whether the garbage collector (GC) must keep outputs of live
>      derivations.
>
>      When set to “yes”, the GC will keep the outputs of any live
>      derivation available in the store—the ‘.drv’ files.  The default is
>      “no”, meaning that derivation outputs are kept only if they are GC
>      roots.  *Note Invoking guix gc::, for more on GC roots.
>
> Specifically this: "derivation outputs are kept only if they are GC roots'".
> Would that mean that other live outputs are deleted?!?  I must get this 
> wrong! :p
>
> That said, I think my `guix-store.el' suggestion still has the following 
> benefit: 
> whenever there is a huge build-input in the store for an item that is only
> seldom rebuilt, it allows the users to chose for themselves if it's OK to 
> delete
> it.

By the way, I think you're right about the manual.  I think it should
say: "The default is 'no', meaning that derivation outputs are kept only
if they are _reachable from_ a GC root."

I've fixed this and added a little more information to the manual in
commit 346a632144b89f45b3966732edacc2bfa07126a7.

> Fantastic review!
> Thank you so much for this: very clear, very thorough, it scattered all
> remaining confusions!  I thumb up for a blog post! :D

I'm glad it helped!  But I must confess, I'm cheating: This is all
described in Eelco Dolstra's thesis [1].  It's a good read if you want
to learn more about how the daemon and the store are designed.

> I find emacs-guix tremendously useful, I highly recommend you give it a try 
> (the
> learning curve is very flat).  Any reason why you'd rather not use it?

I just haven't gotten around to it yet.  I'll try it sooner or later!

>> > While I'm at it, I'd like to note that something might be wrong with the
>> > `-F` option: I never get the promised amount of free space back, only
>> > about 2/3 of it.
>>
>> I think that's normal.  My understanding is that if you say "guix gc -F
>> 3GiB", you aren't asking Guix to free 3 GiB; you're asking Guix to try
>> to free enough space so that when it's done, the store will have 3 GiB
>> of free space.  For example, if the store begins with 2 GiB of free
>> space and there is 5 GiB of garbage, Guix will only collect 1 GiB in
>> order to bring the free space up to 3 GiB.
>
> No, I'm talking total disk usage here.  If I ask 5G back, then `df /` tells me
> that root has about 3-4G free, it never has the promised amount.  Can someone
> confirm this?  Might be a bug.

Perhaps you simply don't have enough garbage available for the collector
to collect?  If you ask for 5 GiB, your system has 3 GiB free, and there
is only 1 GiB of garbage, the best the collector can do is collect all
the garbage (1 GiB) and leave you with just 4 GiB of free space.

Footnotes: 
[1]  Page 128: https://nixos.org/~eelco/pubs/phd-thesis.pdf

-- 
Chris

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